1. Field of the Invention
This invention generally relates to a rewritable thermosensitive recording medium provided with a recording layer, the transparency of which reversibly changes depending on the temperature thereof. More particularly, this invention relates to an improved thermosensitive recording medium that can considerably enlarge the range of a transparent state in terms of the temperature, namely, the range (hereunder referred to simply as a transparency temperature range) of temperature, to which a recording layer should be heated for changing a state thereof from an opaque state to a transparent state, and moreover can hardly cause reduction in contrast between a portion of an image corresponding to an object (hereunder sometimes referred to simply as an object image portion) and the remaining portion (hereunder sometimes referred to simply as a ground image portion) thereof.
2. Description of the Related Art
As is described in the U.S. Pat. Nos. 4,268,413 and 4,695,528, a conventional thermosensitive recording medium of this kind has a structure in which a recording layer containing a thermoplastic resin (for example, polyvinyl chloride) and organic low-molecular-weight materials (for instance, a higher fatty acid) dispersed in the thermoplastic resin as principal components thereof is formed on a support.
Further, this conventional thermosensitive recording medium is made by dexterously utilizing the property of the recording layer that the light absorption characteristic of the recording layer (namely, the intensity of light scattered by the recording layer) varies reversibly depending on the temperature thereof. Namely, in case of this recording layer, there are two state conversion temperatures T.sub.1 and T.sub.2 (incidentally, T.sub.1 &lt;T.sub.2), which are higher than a specific temperature T.sub.0 close to room temperature. When this recording layer is heated to a temperature equal to or higher than the temperature T.sub.2, this recording layer becomes translucent. Thereafter, when this recording layer is cooled to the temperature T.sub.0 or below, this recording layer becomes whitely opaque. Thus the intensity of light scattered by the recording layer increases and the recording layer reaches a maximum light extinction state (namely, a maximum opacity state). In contrast, when this recording layer is heated to a temperature, which is equal to or higher than the state conversion temperature T.sub.1 and lower than that T.sub.2, the intensity of light scattered by this recording layer decreases and thus this recording layer enters a transparent state. Further, such a transparent state is maintained when this recording layer is cooled to the temperature T.sub.0 or below. Moreover, the maximum light extinction state and the transparent state of this recording layer can be maintained at the specific temperature T.sub.0 or below. Furthermore, the state of this recording layer can be reversibly changed between these states. Additionally, an opaque portion of this recording layer (namely, a portion put in the maximum light extinction state) can be distinguished from a transparent portion of this recording layer (namely, another portion put in the transparent state). Therefore, by setting one of these states as a base state (for example, by setting the maximum light extinction state as a base state corresponding to a white ground portion of an image), the conventional thermosensitive recording medium can be used as a rewritable recording medium.
Incidentally, hereinafter, the temperature range between the state conversion temperatures T.sub.1 and T.sub.2 will be sometimes referred to as the transparency temperature-range because the recording layer can be put in the transparent state by heating the recording layer to any temperature of this temperature range.
Such a conventional thermosensitive recording medium, however, has a drawback in that the transparency temperature range is very narrow, namely, 2 to 4 degrees or so in case of employing the centigrade scale and thus the control of the temperature thereof is extremely difficult when the whole or a part of the recording layer, which is in a whitely opaque state (namely, a milky white state), is heated to a temperature equal to or higher than the temperature T.sub.1 and lower than the temperature T.sub.2 to put the recording layer in the transparent state.
Then, in order to enlarge the transparent-state realization temperature-range, the U.S. Pat. No. 4,917,948 has proposed a reversible thermosensitive recording medium in which at least one higher fatty acid having 16 or more carbon atoms and at least one aliphatic saturated dicarboxylic acid or a derivative thereof are employed as the organic low-molecular-weight materials. Further, the U.S. Pat. No. 5,085,934 has proposed another reversible thermosensitive recording medium in which at least one higher fatty acid having 16 or more carbon atoms and at least one aliphatic saturated dicarboxylic acid having 20 or more carbon atoms are employed as the organic low-molecular-weight materials.
Thereby, the transparency temperature-range can be indeed enlarged a little.
However, as is apparent from data on the transparency temperature ranges of comparative examples (to be described later), the enlarged transparency temperature ranges are 20 degrees or so in case of employing the centigrade scale and thus are yet too narrow to completely eliminate the foregoing drawback of the conventional thermosensitive recording medium. Moreover, this results in that the conventional thermosensitive recording medium has another drawback as will be described hereinbelow. Namely, although it is theoretically possible that a transparent portion representing an object is formed in whitely opaque portions representing white grounds in such a thermosensitive recording medium by using a heating means such as a thermal head, the transparency temperature state is narrow yet and thus it is practically difficult to set a heating temperature of the heating means such as a thermal head within the transparency temperature range at a high speed. Therefore, it is usual to perform a method in which the thermosensitive recording medium is first inserted between heating rollers preliminarily heated to a suitable temperature within the transparency temperature range and as a result, becomes uniformly transparent and thereafter a portion of the recording layer is selectively put into the whitely opaque state by using the heating means such as the thermal head preliminarily heated to a temperature equal to or higher than the temperature T.sub.2 and as a result, a whitely opaque portion representing the object is formed in portions representing transparent grounds or bases.
Thus the image formed and recorded by effecting such a method is a reverse image (namely, what is called a negative pattern) of an ordinary picture draft. Therefore, the conventional thermosensitive recording medium has another fatal drawback in that the application thereof is limited to a special technical field.
Moreover, in case of enlarging the transparency temperature range by employing the reversible thermosensitive recording medium proposed in the official gazettes of the U.S. Pat. Nos. 4,917,948 and 5,085,934, as is apparent from data on the transparency temperature range described in the official gazettes of the U.S. Pat. Nos. 4,917,948 and 5,085,934, if a mixing rate of the amount of the aliphatic saturated dicarboxylic acid to that of the higher fatty acid is low, the transparency temperature range is enlarged only a little.
In contrast, as the mixing rate of the amount of the aliphatic saturated dicarboxylic acid to that of the higher fatty acid increases, the transparency temperature range becomes larger. On the other hand, as the mixing rate of the amount of the aliphatic saturated dicarboxylic acid increases and that of the higher fatty acid decreases, the opacity of the object image portion of the recording layer is liable to decrease (see TABLE 2 of U.S. Pat. No. 5,085,934). Therefore, the conventional thermosensitive recording medium has another drawback in that the sufficient contrast between the whitely opaque portion and the transparent portion, which correspond to the object image portion and the ground image portion, respectively, cannot be obtained. The present invention is accomplished to eliminate the foregoing drawbacks of the conventional thermosensitive recording medium.